1. Field of the Invention
This invention relates to a process for the production of cyanuric acid and more specifically to the pyrolysis of urea and its reaction products in two stages, the first stage of the reaction starting with urea in molten form and resulting in a reaction product which comprises about 30% to 60% biuret by weight, and the second stage of which involves further pyrolysis of the first stage reaction product in dry, particulate, solid form to effectively reduce the biuret content and increase the cyanuric acid content to the point of the product being predominantly, up to about 90%, cyanuric acid by weight, and solvent free.
2. Description of the Prior Art
Traditionally, cyanuric acid is produced by heating urea at temperatures between 180.degree. C. and 300.degree. C. according to the following reaction: ##STR1## As urea decomposes, cyanuric acid as well as other pyrolysis products such as biuret, triuret, ammelide, and ammeline are formed with the reaction mass passing from a liquid state to an increasingly viscous slurry. Further heating of the material produces an extremely sticky, solid cake which adheres to equipment surfaces, is difficult to heat, and inhibits the release of ammonia. Various process routes have been used to overcome this handling problem.
Formaini U.S. Pat. No. 3,093,641 discloses a process employing a two-step reaction in which urea is first pyrolyzed to an intermediate reaction product containing about 30% to 50% unreacted urea. In the second step this material, while still molten, is applied as a thin film (less than 0.5 in. thick) to the surface of a rotating drum heated to between 200.degree. C. and 300.degree. C. The Formaini process does provide a means of heat transfer through the sticky, plastic state, but is limited to a rapid reaction time at temperatures above 200.degree. C. in the second stage. The higher temperature of reaction favors the increased production of the by-product ammelide in the resultant product. Ammelide is undesirable in most cyanuric applications and must be removed via acid digestion. In addition the second stage of heating on a rotating drum is not free from problems caused by the product adhering to the equipment. Operating errors can promote major jamming of equipment with accumulated product.
Westfall U.S. Pat. No. 2,943,088 discloses a process whereby cyanuric acid is prepared by heating urea at a temperature of 240.degree. C. to 360.degree. C. while continuously moving it so that it melts and deammoniates through a viscous plastic state into a hard solid state to form a crude reaction product in the form of small granules consisting largely of cyanuric acid, ammelide, ammeline, and minor quantities of other impurities. This patent asserts that urea alone can be used as a feedstock or alternatively, molten urea can be blended with a crude cyanuric product (5% to 35% urea, 95% to 65% cyanuric) to obtain a free flowing mixture which can then be heated in the range of 210.degree. C. to 375.degree. C. to deammoniate the urea through a viscous plastic state to a hard solid state, and moving it through the heated zone while continuously agitating the blend to preserve the granules in free-flowing form. As with Formaini the temperature of reaction cited by Westfall, i.e. above 210.degree. C., tends to generate substantial quantities of impurities, notably ammelide, in the resultant final product. Several examples given by the Westfall patent contain about 40% impurities by weight.
Both the Formaini and Westfall processes have framed the operating parameters of time and temperature, i.e. about 200.degree. C. and up to 200 minutes, so that a very crude cyanuric product is formed. Such products require significant acid digestion to obtain a pure cyanuric acid. Ammelide is an impurity that becomes particularly troublesome in that its chemical structure is similar to cyanuric acid, requiring sophisticated analytical techniques to distinguish between the two. Also, the manufacture of chlorinated isocyanurate products requires that essentially pure cyanuric acid containing a minimum of 95% cyanuric acid be used in the chlorinated step.
Baskin U.S. Pat. No. 3,236,845 discloses a dry process wherein 7-40 parts of cyanuric acid and one part urea are fed through a reactor at 200.degree.-300.degree. C. by helical screw conveyor means with the pyrolysis involving 1-2 minutes transit time. Conveyance can also be by pug mill paddles with a 30 minute residence time at about 280.degree. C.
McBrayer U.S. Pat. No. 3,336,309 discloses a process involving agitating and heating of urea to 432.degree.-450.degree. F. and then to about 535.degree. F.
Ohata et al U.S. Pat. No. 3,953,443 involves agitating and heating of urea to greater than 340.degree. C. and then to greater than 220.degree. C. for over 21/2 hours.
Sato et al U.S. Pat. No. 4,474,957 involves heating cyanuric acid and urea to 260.degree.-270.degree. C. in a kiln with cyanuric acid product recycling.
The present discovery, which is surprising and of great economic benefit, is that reaction products comprising principally cyanuric acid can be produced by a relatively low temperature process with minimal byproduct production of ammelide as an impurity, and with avoidance of the difficulty customarily expected in the handling of a intermediate urea pyrolysis product in its plastic, sticky state. Hithertofore, this has only been accomplished by suspending such intermediate product in a suitable heated liquid solvent.
There are several prior U.S. patents that disclose processes which employ an inert liquid solvent as a heating medium and suspension agent for a feedstock which may include urea, biuret, cyanuric acid, and other homologs, namely U.S. Pat. Nos. 2,822,363, to Christmann, 3,563,987, to Perret, 3,065,233, to Hopkins, 2,872,447, to Ochlschlaeger, 3,008,961, to Wojcik, 3,563,987, to Berkowitz, 3,172,886, to Christoffel, and 3,164,591 to Walles. Heat transfer and removal of evolved ammonia is very good in such systems and adhesion of product to equipment surfaces is lessened. Since the plastic phase of the reaction mass which in general corresponds to an analysis of 21% to 29% urea is overcome by dispersion in a solvent, the temperature and time of reaction in a solvent process is quite arbitrary, being in a range of 150.degree. C. to 300.degree. C. and generally from 1 to 10 hours. Product yields can be quite extraordinary as evidenced by the process disclosed in U.S. Pat. No. 3,563,987, to Berkowitz which asserts a purity as high as 99.7% cyanuric acid with a yield of 97%. The generation of ammelide can be reduced by using operating temperatures about 200.degree. C. or less in combination with vacuum or a gas sweep to rapidly remove released ammonia from the reaction mass, but only at the added cost of the expensive solvent unavoidably lost in the vapor phase. Another major limitation in the solvent approach is the filtration step required to separate the cyanuric acid from the solvent. This step of necessity often generates a waste stream of such solvents as dipropylene glycol, phenols, alkylsufones, tetrahydrofurfuryl alcohol, or trichlorobenzene which must be recovered. Other problems include emission of the solvent in the ammonia offgas stream, making the ammonia unsuitable for some end uses such as a nitrogen source in fertilizers, and the breakdown of the solvent in repeated heating cycles. All of the solvents suggested also pose exposure hazards to operating personnel and require significant operating procedures for environmental and safety reasons.
U.S. Pat No. 4,093,808 to Nelson involves agitating and heating cyanuric acid with a minor amount of nitric acid or ammonium nitrate to 250.degree.-300.degree. C.
The prior art patents most closely related to the present invention are believed to be Stephan et al U.S. Pat. Nos. 4,540,820 and 4,654,441. These patents disclse a two step process for the production of biuret in a form suitable for use as animal feed, comprising at least 55% biuret by weight. The preparation of the feedstock, i.e. a first phase intermediate reaction product by controlled pyrolysis of urea, is generally quite similar for the feed grade biuret process and for process of the present invention, and it is to be noted that both the earlier Stephan et al biuret production process and the present process for formation of a predominantly cyanuric acid product employ a second stage of heating of a comminuted solid product in a forced air recirculating oven. Example 3 of U.S. Pat. No. 4,654,441 indicates that a product containing as much as 42.5% cyanuric acid by weight can result from the earlier process, which product, however, is not suitable for use as animal feed grade biuret. However, neither of the cited prior Stephan et al patents discloses any teaching with regard to attaining products comprising more than 50% cyanuric acid by weight, or how to effectively realize such products. The Stephan et al biuret production processes, as disclosed in the noted prior Stephan et al patents, emphasize minimal conversion of the urea and biuret to cyanuric acid and, in actual operating practice (in keeping with the FDA requirement of less than 30% cyanuric acid in animal feed grade biuret), require second stage heating at less than 130.degree. C.
Heretofore, cyanuric acid production processes which operate without a solvent and which produce a relatively high purity cyanuric acid with minor amounts of ammelide have been difficult to attain. Prior art teachings suggest that temperatures above 200.degree. C. are necessary for significant non-solvent conversion to cyanuric acid. Experience has shown, however, that heating of the urea/biuret Stephan et al first stage intermediate reaction product above 200.degree. C. results in excessive production of ammelide, severe loss of product through sublimation of the urea, biuret, and other intermediates, and shrinkage of the product bed in the second stage of heating which causes uneven conversion of cyanuric acid as a result of uneven exposure of the reactants to air. What is surprising and the object of this invention is that an intermediate biuret feed stock can be converted to predominantly cyanuric acid with minor amounts of ammelide at temperatures of 200.degree. C. or less without incurring substantial losses in sublimed product or without rendering the second stage heating of the solid comminuted product ineffective. As a practical matter, it is important to maintain the level of heating at not more than the temperature at which the comminuted second stage reactants soften, so as not to reduce reactant surface area and consequent reduction in contact thereof with heated air. As a practical matter, also, it is important, for a commercial process in which the cyanuric acid content of the reaction product is to be maximized consistent with practical production times, that the comminuted product from the first stage of urea/biuret pyrolysis be such that the urea content of the feedstock to the second stage be less than about 40% by weight and the biuret content thereof be more than about 30% by weight so that the second stage heating by heated air flow through the comminuted feedstock can proceed at an effective temperature and be progressively increased fairly rapidly without melting of the communited product bed, with higher temperatures approaching 200.degree. C. during the latter phases of the second stage reaction being effective to maximize the cyanuric acid content of the final reaction product.
As indicated, the process of the present invention does not employ a solvent and thereby avoids the filtration step required to separate solvent and product and the environmental problems inherent in use of a solvent. Also, the ammonia offgas can be captured as a pure stream that is not contaminated with solvent vapor. The present invention still however retains the benefits of a low temperature conversion process that selectively favors the formation of cyanuric acid. The problem of adhesion of product to equipment is also totally bypassed. In this way the process of the present invention overcomes the limitations inherent with previous nonsolvent processes, which involve higher levels of impurities in the course of the conversion to cyanuric acid and do not totally eliminate sticking problems, and is also an improvement over the solvent processes by eliminating the negative consequences of solvent use.